define isort first party (app and tests)

we were seeing isort produce different outputs locally and in docker -
this was due to it having different opinions about whether the tests
module (ie all our unit tests) is a first party (local) or third party
(pip installed) import. It's a first party import, so by defining this
in the setup.cfg isort settings, we can force it to be consistent
between environments.

Note: I don't know why it was different in the first place though

tests/app/main/views/test_service_settings.py

@@ -5,6 +5,9 @@ import pytest
 from bs4 import BeautifulSoup
 from flask import url_for
 from freezegun import freeze_time
+
+import app
+from app.utils import email_safe
 from tests import service_json, validate_route_permission
 from tests.conftest import (
     SERVICE_ONE_ID,
@@ -28,9 +31,6 @@ from tests.conftest import (
     platform_admin_user,
 )
 
-import app
-from app.utils import email_safe
-
 
 @pytest.fixture
 def mock_get_service_settings_page_common(